common: introduce GITERR_CHECK_ALLOC_BUF

We commonly have to check if a git_buf has been allocated
correctly or if we ran out of memory. Introduce a new macro
similar to `GITERR_CHECK_ALLOC` which checks if we ran OOM and if
so returns an error. Provide a `#nodef` for Coverity to mark the
error case as an abort path.

script/user_nodefs.h

@@ -6,6 +6,7 @@
  */
 
 #nodef GITERR_CHECK_ALLOC(ptr) if (ptr == NULL) { __coverity_panic__(); }
+#nodef GITERR_CHECK_ALLOC_BUF(buf) if (buf == NULL || git_buf_oom(buf)) { __coverity_panic__(); }
 
 #nodef GITERR_CHECK_ALLOC_ADD(out, one, two) \
 	if (GIT_ADD_SIZET_OVERFLOW(out, one, two)) { __coverity_panic__(); }

src/common.h

@@ -89,6 +89,11 @@
  */
 #define GITERR_CHECK_ALLOC(ptr) if (ptr == NULL) { return -1; }
 
+/**
+ * Check a buffer allocation result, returning -1 if it failed.
+ */
+#define GITERR_CHECK_ALLOC_BUF(buf) if ((void *)(buf) == NULL || git_buf_oom(buf)) { return -1; }
+
 /**
  * Check a return value and propagate result if non-zero.
  */