shared/specifier: be less extravagant with memory allocations

ubsan times out because we do too many allocations:

$ valgrind build/fuzz-unit-file test/fuzz-regressions/fuzz-unit-file/oss-fuzz-6977-full
...
test/fuzz-regressions/fuzz-unit-file/oss-fuzz-6977-full... ok
==1757==
==1757== HEAP SUMMARY:
==1757==     in use at exit: 0 bytes in 0 blocks
==1757==   total heap usage: 199,997 allocs, 199,997 frees, 90,045,318,585 bytes allocated

...
==3256==   total heap usage: 100,120 allocs, 100,120 frees, 13,097,140 bytes allocated

https://oss-fuzz.com/v2/issue/4651449704251392/6977 should now be really fixed.

e3c3d6761b was the first attempt, but even with this change, e3c3d6761b
still makes sense.

src/shared/specifier.c

@@ -46,7 +46,7 @@
 #define POSSIBLE_SPECIFIERS ALPHANUMERICAL "%"
 
 int specifier_printf(const char *text, const Specifier table[], void *userdata, char **_ret) {
-        size_t l;
+        size_t l, allocated = 0;
         _cleanup_free_ char *ret = NULL;
         char *t;
         const char *f;
@@ -57,14 +57,11 @@ int specifier_printf(const char *text, const Specifier table[], void *userdata,
         assert(table);
 
         l = strlen(text);
-        ret = new(char, l+1);
-        if (!ret)
+        if (!GREEDY_REALLOC(ret, allocated, l + 1))
                 return -ENOMEM;
-
         t = ret;
 
-        for (f = text; *f; f++, l--) {
-
+        for (f = text; *f; f++, l--)
                 if (percent) {
                         if (*f == '%')
                                 *(t++) = '%';
@@ -77,7 +74,6 @@ int specifier_printf(const char *text, const Specifier table[], void *userdata,
 
                                 if (i->lookup) {
                                         _cleanup_free_ char *w = NULL;
-                                        char *n;
                                         size_t k, j;
 
                                         r = i->lookup(i->specifier, i->data, userdata, &w);
@@ -87,14 +83,9 @@ int specifier_printf(const char *text, const Specifier table[], void *userdata,
                                         j = t - ret;
                                         k = strlen(w);
 
-                                        n = new(char, j + k + l + 1);
-                                        if (!n)
+                                        if (!GREEDY_REALLOC(ret, allocated, j + k + l + 1))
                                                 return -ENOMEM;
-
-                                        memcpy(n, ret, j);
-                                        memcpy(n + j, w, k);
-
-                                        free_and_replace(ret, n);
+                                        memcpy(ret + j, w, k);
                                         t = ret + j + k;
                                 } else if (strchr(POSSIBLE_SPECIFIERS, *f))
                                         /* Oops, an unknown specifier. */
@@ -110,7 +101,6 @@ int specifier_printf(const char *text, const Specifier table[], void *userdata,
                         percent = true;
                 else
                         *(t++) = *f;
-        }
 
         /* if string ended with a stray %, also end with % */
         if (percent)