Fix some compiler warnings (#289)

* Fix comparison of non-dereferenced char* with '\0'

* Fix left-shifting of signed negative integers

It produced lots of compiler warnings on UB on code such as
MMC_IMMEDIATE(MMC_TAGFIXNUM(-1)) (and possibly other instances
of UB not detected by the compiler).

OMCompiler/SimulationRuntime/c/meta/meta_modelica_data.h

@@ -138,15 +138,15 @@ typedef int mmc_switch_type;
 #define MMC_TAGPTR(p)             ((void*)((char*)(p) + 3))
 #define MMC_UNTAGPTR(x)           ((void*)((char*)(x) - 3))
 #define MMC_IS_INTEGER(X)         (0 == ((mmc_sint_t) (X) & 1))
-#define MMC_TAGFIXNUM(i)          (((i) << 1)+0)
+#define MMC_TAGFIXNUM(i)          ((((mmc_uint_t) (i)) << 1)+0)
 #define MMC_UNTAGFIXNUM(X)        (((mmc_sint_t) (X)) >> 1)
 
 #else
 
 #define MMC_TAGPTR(p)             ((void*)((char*)(p) + 0))
 #define MMC_UNTAGPTR(x)           ((void*)((char*)(x) - 0))
 #define MMC_IS_INTEGER(X)         (1 == ((mmc_sint_t) (X) & 1))
-#define MMC_TAGFIXNUM(i)          (((i) << 1)+1)
+#define MMC_TAGFIXNUM(i)          ((((mmc_uint_t) (i)) << 1)+1)
 #define MMC_UNTAGFIXNUM(X)        (((mmc_sint_t) ((X)-1)) >> 1)
 
 #endif

OMCompiler/SimulationRuntime/c/simulation/simulation_info_json.c

@@ -110,7 +110,7 @@ static const char* skipValue(const char* str)
       switch (*str) {
       case '\0': fprintf(stderr, "Found end of file, expected end of string"); abort();
       case '\\':
-        if (str+1 == '\0') {
+        if (*(str+1) == '\0') {
           fprintf(stderr, "Found end of file, expected end of string"); abort();
         }
         str+=2;